Transcript for Before American Murdered By ISIS, US Launched Doomed Rescue Op

Good evening, tonight, the global manhunt is on for the world's most wanted men, the islamic terrorist responsible for executing American journalist, James Foley, just hours ago, the FBI director claimed the full force of the agency will be brought to bear on the terrible, senseless killing. But who are the jihadists spilling hate with an English accent? More on our series "Nightline" investigating. Reporter: As outrage spread around tell world today over this scene, Jim Foley's brokenhearted parents said they had never been prouder of their son. He was strong, courageous, loving to the end, I mean, we just hardly recognize our little boy. I mean, he was just a hero. And you know from the videos that his last words were, I wish I had more time -- to see my family. Reporter: As Foley's family mourns their loss, president Obama called for the world to finally stand up to the threat of the brutal Isis group. No just god would stand for what they do every single day. Reporter: The U.S. Was looking for clues on the identity of the hooded executioner of Jim Foley, using a data base to look for his eyes, hands, and most telling his voice with what british authorities call a distinctive London accent. And go far out of the way to find reasons. From what we see it looks increasingly likely that it is a british citizen. This is deeply shocking, but we know that far too many british citizens have traveled to Iraq and Syria and take part in the extremism and violence. Reporter: In the last year, foreign fighters from Britain, elsewhere and the U.S. Have helped Isis conduct a well-funded, well-armed brutal holy war. Seizing large areas of Syria and Iraq where Christians and non-believers have been executed, some beheaded and even killed. In some ways they are even more extreme than al-qaeda. If somebody says oh, it is a long way. There is no threat to us. It is a threat, they are dangerous and after us. Reporter: Especially with so many western recruits, many from the streets of London. What is it you hope to achieve tonight. Reporter: It is a place that was reported earlier this year, young muslims conducted sharia patrols to enforce stricter sharia law. A bad thing, a noble aim. But sharia, sadly, I think that will be the beginning of the end of the United States of America or Britain and Europe. And Taiwan is currently being governed by man made laws. Reporter: And over the last few months a growing number of british recruits have appeared in Isis propaganda videos. To all of my brothers living in the west, I know how you feel, in your hearts, you feel depressed. Come to jihad and feel what we are feeling. Reporter: James Foley learned that firsthand, a former hostage was held with Foley in a dark cell for some several months told ABC news that Foley was often singled out for tough punishment, once for planning an escape. James, for being an American, and if he kept to his fundamentals that he had, and a brother serving in the U.S. Air force got unfortunately a bit of a special treatment. He was kind of a scapegoat. So every time they would be around, the kicks and punches, he would get a bit more than the others typically. Reporter: But anan also said that Foley was never afraid even in the darkest hours. He was always very strong. And I -- I could see that he -- he gave luck to lots of prisoners. Every time we had a setback he was telling us, oh, don't worry, mate. Reporter: There was no further word today about the other American hostage shown on the video. Steven sotloff, who Isis also threatened to kill if the bombing in Iraq continued. President Obama said the U.S. Would not be intimidated. We will be vigilant and relentless, when people harm Americans everywhere we do everything we can to see that justice is done. The battle for the country's largest city is also taking a deadly toll on citizens. Reporter: Foley filed his report from his global post just before he was captured almost two years ago. This location in aleppo, James Foley for global post. Reporter: In his final moments, James Foley faced death with a courage that marked his journalistic career. And tonight, details if they had worked would have saved his life. Senior officials tell ABC news that president Obama authorized a bold, U.S. Military operation into Syria earlier this summer to rescue Foley and several other U.S. Hostages. But then when the special operations attempt arrived at the site there were no hostages at the site. Officials say several U.S. Forces were involved and several Isis fighters were killed. There were no other casualties, and why the intelligence was not good enough. I know we'll continue to report on that. As always, thanks, Brian Ross for your extensive reporting.

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